1. Field of the Invention
The present invention generally relates to voltage to current converter circuits suitable for driving high-inductance loads and, more particularly, to high speed, high precision coil drivers for electron beam lithography tools, precision cathode ray tubes and precision positioning systems.
2. Description of the Prior Art
Supplying accurate levels of voltage or current to a reactive load presents substantial difficulty when the rate of voltage or current change is large. This circumstance is present and the difficulty particularly evident in arrangements for precision positioning devices (e.g. translation tables) or electromagnetic deflection of a charged particle beam where the beam must be deflected to impinge on different and often widely separated locations of the target at high speed. High speed of deflection or motion is necessary in cathode ray tubes to avoid luminance artifacts, such as image flickering, from being developed. In electron beam lithography tools, high deflection speed and positional accuracy with very short settling time is necessary to maintain acceptable levels of exposure throughput.
In high resolution lithography applications, charged particle electrostatic deflectors are useful for high speed and small size vector deflectors (e.g. 1-10 MHz and under 50 xcexcm radius). Electron optical aberrations and deflection voltages increase greatly as the electrostatic deflection range is increased. Also, electrostatic deflection plates can be unreliable because of contamination from hydrocarbon deposits which are an unwanted but unavoidable byproduct of photoresist exposure. Such hydrocarbon deposits can build up over time and form an insulating layer that can be charged by the charged particle beam. This unwanted and unpredictable charge on the contaminated deflection plates can produce undesired and unpredictable deflections.
Therefore, magnetic deflection is generally preferred to electrostatic deflection for slower speed and larger range deflections and are not subject to unreliability from contamination since they are generally placed outside the vacuum envelope. Correction of aberrations is also well-understood and well-developed at the present time. However, electromagnetic deflection yokes present a very large inductive load which must be driven with high current.
Of course, image errors are cumulative and effects of undershoot, overshoot and ringing must be held to a small fraction of the resolution of the overall electron beam exposure system. By the same token, recent improvements in resolution have greatly increased the accuracy with which beam deflection must be accomplished. Accordingly, it can be readily understood that previously adequate deflection driver circuits must necessarily compromise image fidelity (by introducing deflection errors comparable to or larger than the resolution of the system) or throughput (by increasing the time required to step and settle).
A complicating factor in design of drivers for inductive loads is that amplifier circuits therein can easily become saturated. High accuracy is generally achieved through feedback and rapid change of input signal can cause large error signals that can drive amplifier output to substantially the power supply voltage at which some latching effects are observed. Transients and ringing can persist for many microseconds as the servo or feedback loop regains stability after a hard saturation.
Another complicating factor is drift of amplifier or drive circuit output with temperature. Since currents in yokes of electromagnetic deflection systems are large, temperature excursions will be substantial. Since the response of an amplifier (or any other active semiconductor device) to a given input signal will change significantly with temperature, feedback arrangements are generally subject to drift since the error signal will differ from the signal necessary to obtain the required output response and additional corrective circuitry must be provided to compensate for thermal drift. Of course, such corrective circuitry increases complexity and cost and correction to accuracies now required may not be practical.
Additionally, the resistive component of the load impedance may drift with temperature as is particularly likely in charged particle beam deflection systems where coil current is very high. The driver must also provide compensation for this change in impedance such that the current in the inductive load is not altered significantly from the intended levels. As with thermal drift of the driver circuit, such compensation requires additional complexity and cost and is complicated by the possibility of thermal drift discussed above.
Accordingly, it is seen that known driver circuits for reactive loads cannot provide the accuracy of output required for driving electromagnetic charged particle beam deflection yokes with the required accuracy at the required deflection speeds and require a trade-off between these operating parameters. Further, compensation for thermal drift to accuracies now required may not be possible, particularly in a manner consistent with simultaneous requirements for deflection accuracy and speed.
It is therefore an object of the present invention to provide a driver circuit for large reactive (e.g. inductive) loads which achieves an accuracy of better than a very few parts per million in response to inputs which can change very rapidly.
It is another object of the invention to provide a driver circuit for large reactive loads which can maintain thermal drift within the limits of accuracy of a very few parts per million.
It is a further object of the invention to provide a driver circuit for large reactive loads which can be adjusted to achieve critical damping to reduce undershoot, overshoot and ringing to negligible levels within extremely short periods of time.
In order to accomplish these and other objects of the invention, a circuit for driving an inductive load is provided comprising a first amplifier and a second amplifier connected in cascade, said second amplifier being subject to offset drift with temperature variation, open loop gain of the first amplifier being much greater than a gain of the second amplifier, an arrangement for sensing an output of the second amplifier and providing a feedback signal to an input of the first amplifier, and an arrangement for thermally isolating or regulating temperature of the first amplifier.